This invention relates to dewatering of coal slurries, and in particular to a process and plant for the production of hardened pellets from a coal slurry.
Coal may be classified according to its size. Particles larger than, say, 25 mm are generally referred to as "lumps". Between 25 mm and, say 150 microns the particles may be referred to as "grains", and below 150 microns as "fines". The actual dividing lines are not firmly fixed; for instance, that separating the grains from the fines may lie anywhere between 75 microns and 250 microns, though more usually it is taken at 100 or 150 microns.
This specification is mainly concerned with grains and fines, since it is that these are most usually found in coal slurries, such as those used for pipeline transport. Larger lumps are difficult to transport as a slurry as they require a large energy input to keep them in suspension; fines on the other hand lend themselves to such pipeline transport, but are not easy to dewater.
One technique which is suitable for dewatering fines is agglomeration using a binder. The resulting agglomerates can then be separated mechanically from water and any non-agglomerated matter by, for example, filtration or centrifuging. Depending on the method of fabrication, the agglomerates can be made into quite hard pellets, but "green" pellets, i.e. those which have not undergone some heat treatment or curing, are weak and may break up during handling. Curing of the pellets requires a heat input which adds to the cost of the processing; it can also lead to the production of noxious odors and fumes.
U.S. Pat. No. 1,589,023 relating to a process for cracking coal and oil has some pertinence to the invention.